The USA has a national holiday called Thanksgiving Day on the 4th Thursday in November. TAR seems to like to start filming on the Sat after this holiday. I suspect work rules concerning overtime pay for the crew is probably pretty high if they are actually working on that holiday. Since TAR has gotten very good at filming a whole season in 21-23 days....that gives them time to film a season between Thanksgiving and Christmas and not have to pay that overtime penalty.

The one element we can't be sure of is when the filming for a particular locale has to take place at a particular time, or that they need to avoid at a particular time due to national or cultural factors. This was never more clear than in TARAS when teams were delayed traveling in Africa due to travel for a religious holiday or festival that congested flights all over the African continent.

That's an element that is hard to predict unless we know the itinerary beforehand (boy wouldn't that be fun!) but it is a factor that TPTB have learned they needed to deal with in planning, and that might dictate an earlier or later start to filming.